Re: [PATCH v2 1/2] Provide in-kernel headers for making it easy to extend the kernel

[Masahiro Yamada]

On Fri, Feb 15, 2019 at 11:48 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Mon, Feb 11, 2019 at 09:35:59AM -0500, Joel Fernandes (Google) wrote:
> > Introduce in-kernel headers and other artifacts which are made available
> > as an archive through proc (/proc/kheaders.txz file).


The extension '.txz' is not used in kernel code.


'.tar.xz' is used for 'tarxz-pkg', 'perf-tarxz-src-pkg' etc.


$ git grep  '\.txz'
$ git grep  '\.tar\.xz'
Documentation/admin-guide/README.rst:     xz -cd linux-4.X.tar.xz | tar xvf -
arch/x86/crypto/camellia-aesni-avx-asm_64.S: *
http://koti.mbnet.fi/axh/crypto/camellia-BSD-1.2.0-aesni1.tar.xz
crypto/testmgr.h: *   https://bench.cr.yp.to/supercop/supercop-20170228.tar.xz
crypto/testmgr.h: *   https://bench.cr.yp.to/supercop/supercop-20170228.tar.xz
crypto/testmgr.h: *   https://bench.cr.yp.to/supercop/supercop-20170228.tar.xz
crypto/testmgr.h: *   https://bench.cr.yp.to/supercop/supercop-20170228.tar.xz
crypto/testmgr.h: *   https://bench.cr.yp.to/supercop/supercop-20170228.tar.xz
scripts/package/Makefile:       @echo '  perf-tarxz-src-pkg  - Build
$(perf-tar).tar.xz source tarball'
tools/testing/selftests/gen_kselftest_tar.sh:
 ext=".tar.xz"



I prefer '.tar.xz' for consistency.






BTW, have you ever looked at scripts/extract-ikconfig?

You added IKHD_ST and IKHD_ED
just to mimic kernel/configs.c

It is currently pointless without the extracting tool,
but you might think it is useful to extract headers
from vmlinux or the module without mounting procfs.




> > This archive makes
> > it possible to build kernel modules, run eBPF programs, and other
> > tracing programs that need to extend the kernel for tracing purposes
> > without any dependency on the file system having headers and build
> > artifacts.
> >
> > On Android and embedded systems, it is common to switch kernels but not
> > have kernel headers available on the file system. Raw kernel headers
> > also cannot be copied into the filesystem like they can be on other
> > distros, due to licensing and other issues. There's no linux-headers
> > package on Android. Further once a different kernel is booted, any
> > headers stored on the file system will no longer be useful. By storing
> > the headers as a compressed archive within the kernel, we can avoid these
> > issues that have been a hindrance for a long time.
>
> The set looks good to me and since the main use case is building bpf progs
> I can route it via bpf-next tree if there are no objections.
> Masahiro, could you please ack it?


Honestly, I was not tracking this thread
since I did not know I was responsible for this.


I just started to take a closer look, then immediately got scared.

This version is not mature enough for the merge.



First of all, this patch cannot be compiled out-of-tree (O= option).


I do not know why 0-day bot did not catch this apparent breakage.


$ make -j8 O=hoge
make[1]: Entering directory '/home/masahiro/workspace/bsp/linux/hoge'
  GEN     Makefile
  Using .. as source for kernel
  DESCEND  objtool
  CALL    ../scripts/checksyscalls.sh
  CHK     include/generated/compile.h
make[2]: *** No rule to make target 'Module.symvers', needed by
'kernel/kheaders_data.txz'.  Stop.
make[2]: *** Waiting for unfinished jobs....
/home/masahiro/workspace/bsp/linux/Makefile:1043: recipe for target
'kernel' failed
make[1]: *** [kernel] Error 2
make[1]: *** Waiting for unfinished jobs....
make[1]: Leaving directory '/home/masahiro/workspace/bsp/linux/hoge'
Makefile:152: recipe for target 'sub-make' failed
make: *** [sub-make] Error 2





I was able to compile it in-tree
but it makes the incremental build extremely slow.

(Here, the incremental build means
"make" without changing any code after the full build.)




Before this patch, "make -j8" took 11 sec on my machine.

real 0m11.777s
user 0m16.608s
sys 0m5.164s



After this patch, x86_64_defconfig + CONFIG_IKHEADERS_PROC=y
takes 53 sec for me since kernel/kheaders_data.txz is regenerated
every time even when you did not touch any source file.


$ time make -j8
  DESCEND  objtool
  CALL    scripts/checksyscalls.sh
  CHK     include/generated/compile.h
  GEN     kernel/kheaders_data.txz
  UPD     kernel/kheaders_data.h
  CC      kernel/kheaders.o
  AR      kernel/built-in.a
  GEN     .version
  CHK     include/generated/compile.h
  UPD     include/generated/compile.h
  CC      init/version.o
  AR      init/built-in.a
  AR      built-in.a
  LD      vmlinux.o
  MODPOST vmlinux.o
  KSYM    .tmp_kallsyms1.o
  KSYM    .tmp_kallsyms2.o
  LD      vmlinux
  SORTEX  vmlinux
  SYSMAP  System.map
  Building modules, stage 2.
  CC      arch/x86/boot/version.o
  MODPOST 17 modules
  VOFFSET arch/x86/boot/compressed/../voffset.h
  OBJCOPY arch/x86/boot/compressed/vmlinux.bin
  RELOCS  arch/x86/boot/compressed/vmlinux.relocs
  CC      arch/x86/boot/compressed/kaslr.o
  GZIP    arch/x86/boot/compressed/vmlinux.bin.gz
  CC      arch/x86/boot/compressed/misc.o
  MKPIGGY arch/x86/boot/compressed/piggy.S
  AS      arch/x86/boot/compressed/piggy.o
  LD      arch/x86/boot/compressed/vmlinux
  ZOFFSET arch/x86/boot/zoffset.h
  OBJCOPY arch/x86/boot/vmlinux.bin
  AS      arch/x86/boot/header.o
  LD      arch/x86/boot/setup.elf
  OBJCOPY arch/x86/boot/setup.bin
  BUILD   arch/x86/boot/bzImage
Setup is 15612 bytes (padded to 15872 bytes).
System is 12673 kB
CRC 697aaf88
Kernel: arch/x86/boot/bzImage is ready  (#6)

real 0m53.024s
user 0m32.076s
sys 0m9.296s




Also, I notice $(ARCH) must be fixed to $(SRCARCH),
but that is one of minor issues.



We should take time for careful review and test.


Please give me more time for thorough review.



--
Best Regards
Masahiro Yamada